1. Field of the Invention
The present disclosure relates generally to the fields of ionic liquids, lubricant compounds, osseointegrative compounds, antimicrobial compounds, biomedical alloys, and surface modification. In particular, the disclosure relates to ionic liquids which can be deposited on devices.
2. Description of Related Art
Titanium and its alloys are broadly used in the design of dental and orthopedic implants due to a combination of attractive properties that include high corrosion resistance, biocompatibility, passivation and adequate mechanical properties. However, orthopedic implants are normally subject to wear and friction because they lack the body natural lubrication, which keeps joints immune to wear (Streicher, 1998). Modular implants designed with metal-on-metal (MoM) connections are rather popular in orthopedic and oral prostheses nowadays given their flexibility for matching patient anatomy and ease of intraoperative adjustments. However, such MoM modular connections are prone to micromotion and have been reported to wear and undergo corrosion in the absence of lubrication. This phenomenon, known as fretting-crevice corrosion, will ultimately lead to exposure of the bulk metal to active dissolution. Dissolution and consequent leaching of metal ions and debris in vivo can trigger inflammatory responses and adverse tissue reactions. Retrieval studies have shown several cases of implant failure as a consequence of the conjoint effects of corrosion and fretting (Gilbert et al., 1993; Gilbert et al., 1997; Jacobs et al., 1998; Goldberg et al., 2002). Collier et al. (1991) found that 17 of 30 mixed-metal femoral prostheses presented time dependent evidence of corrosion, in which the crevice provided between the head and neck connection functioned as corrosion sites due to the development of stagnant aqueous environments at those interfaces. The use of MoM in the articulation interfaces between the acetabular cup and femoral head of hip implants has been recently decreased due to reports of high failure rates and adverse local tissue reaction including pseudotumors and peri-fluid collections. Modularity in other implant areas such as the head-neck, stem-stem and stem-neck are also sources of corrosion and wear (Rodrigues et al. 2009). Corrosion has also been observed with titanium dental implants, and has been suggested to be one of triggering factors for peri-implantitis. Current designs of dental implants containing a modular abutment and smooth collar are also subject to corrosion because those areas are highly exposed to the oral environment, which can become acidic in the event of inflammation or presence of bacterial biofilms (Rodrigues et al., 2013). Bacterial biofilms can get adhered to the surface of a dental implant interrupting the process of osseointegration, which can lead to implant loosening and failure. These observations show that implant modularity can lead to corrosion. The presence of bacteria in the oral environment, for example, can lead to a significant reduction of the environment pH, which can trigger surface corrosion. Besides providing the conditions to trigger oxidation, bacterial biofilms can get adhered to the surface of implants interrupting surface aeration and leading to the permanent breakdown of the oxide layer, which will disrupt integration of the implant with bone.
Titanium and its alloys (Ti) form an oxide layer in presence of oxygen that will ensure bioactivity and protection against corrosion. Because the bioactivity of Ti oxides is not sufficient to provide anchorage of an implant, several strategies have been developed to modify the surface of the material to achieve integration with bone (Wang et al., 2011). Osseointegration is an important step to ensure implant success; this is especially important with dental implants. When the surface of a dental implant is well-integrated, it will provide mechanical stability and prevent entrance/leakage of bacteria (Svensson et al., 2013). It has been discussed in the literature that biofilm adhesion on the surface of dental implants and/or mechanical overload can hinder osseointegration and cause implant loosening (Renvert et al., 2007; Simchi et al., 2011; Zemmerli et al., 2006; Davis, 2003; Tillander et al., 2010; Quirynen et al., 1993; Quirynen et al., 2002). The scenario is similar with orthopedic implants, where the more porous exterior surfaces will achieve better integration.
Concerns related to infection with dental and orthopedic implants have led many researchers to explore coatings that can deliver antimicrobial/antibiotic compounds (Simchi et al., 2011; Svensson et al., 2013; Tsuchiya et al., 2012; Holmberg et al., 2013; Vargas-Reus et al., 2012; Cheng et al., 2013; Narbat et al., 2012) against biofilm adhesion. Thus, osseointegrative and antimicrobial activity are desirable surface characteristics. Surface modification of implants performed via chemical, physical and electrochemical approaches have produced diverging results. Surface macro- and micro-texturing achieved by methods such as grit-blasting, plasma spraying, sintered beads, fiber meshes have been reported to produce heterogeneous coatings, degradation, delamination and particle release over time (Wang et al., 2011, Ferris et al., 1999; Gibson and Stamn, 2012). Current antimicrobial coatings prepared using specific chemistries such as silver, iodine, organosilane compounds and nitric oxide, have produced inconsistent results (Simchi et al., 2011; Tsuchiya et al., 2012).
Given the current trend with implant designs, novel methods for surface protection of implant contacting interfaces are of great importance. Typical surface treatments are not applicable to the irregular modular areas of orthopedic implants because they alter surface roughness or do not promote the formation of homogenous layers on the substrate. These rough coatings have also been reported to fail releasing fragments in the body. Ionic liquids (ILs) are currently under investigation as lubricant for applications where conventional oils and other lubricants are not applicable. Jimenez and Bermudez (2009) have studied the use of ILs as lubricants of high temperature metals and alloys such as titanium and nickel. Ionic liquids are molten salts, which possesses a combination of excellent properties such as non-volatility, non-flammability, thermo-oxidative stability, high ionic conductivity, wide electrochemical window, and miscibility with organic compounds (Bermudez et al., 2009; Minami, 2009; Liu et al., 2002). They were demonstrated to have good lubrication and wear resistant properties for materials that slide against each other, as in the case of steel-steel and titanium-steel counterparts (Jimenez and Bermudez, 2009). ILs are usually composed of an organic cation, typically containing nitrogen or phosphorous, and a weekly coordinating anion. Some of the most common cations employed are imidazolium, phosphonium, pyridinium and ammonium, while some common anions are BF4−, PF6−, CF3SO3− and N(CF3SO2)2− (Bermudez et al., 2009). The anions have a significant influence on the tribological properties of ILs, for example, anionic moieties should be hydrophobic to improve substrate-IL properties. When there is formation of complexes between substrates and IL molecules, adsorbed IL layers become stable protecting the surface (Bermudez et al., 2009). The excellent tribological properties of IL additives are attributed to the formation of physically adsorbed films on different substrates.
Because of the need for improved lubrication of implant modular connections, protection of the surface against biofilm formation, while providing a permissive environment for host cell integration (soft and hard tissue integration) for successful implant fixation, the development of new surface modification techniques is of importance. This disclosure provides new multifunctional ionic liquids which in some aspects can be used for surface modification of biomedical implants.